The present invention relates to an improved process for the ionic polymerisation of ethylene under high pressure and more particularly to a high temperature polymerisation process which makes it possible to manufacture high-density polyethylene having a melt index greater than 0.1 and a polydispersity index greater than that obtained by the known processes. As used in the present application, polydispersity index denotes a number which measures the molecular weight distribution and is equal to the ratio (Mw/Mn) of the weight-average molecular weight to the number-average molecular weight, these weights being measured by gel permeation chromatography.
It is well-known to polymerise ethylene under high pressure at a high temperature by an ionic mechanism, using a catalyst system of the Ziegler type which comprises, firstly, a halogen compound of a transition metal such as titanium trichloride and, secondly, an organo-aluminum activator such as a trialkyl-aluminium or alkyl-siloxalane. However, the manufacture, by this prior process, of high-density polyethylene having a melt index greater than 0.1 (measured according to Standard Specification ASTM D 1238-73) and having a high polydispersity index, is particularly difficult. In fact, this prior process much more easily gives high-density polyethylene of melt index greater than 1 and of polydispersity index of less than 6, which is very suitable for processing by injection-moulding. In particular, adapting only the temperature and pressure conditions is not sufficient to change the index of (Mw/Mn) in the desired sense; the use of a reactor comprising several reaction zones which operate at different temperatures, or the arrangement of several reactors in series or in parallel permits only an insufficient broadening of the molecular weight distribution of the resin produced.
Similarly, the process of polymerisation in the presence of hydrogen, described in British Pat. No. 1,419,012, which consists in not introducing the gas phase from the separator into the first zone of the reactor, is of limited efficiency because the feed rate of ethylene of low hydrogen content cannot exceed about 20% of the total feed rate.